Photoalignment of a Nematic Liquid Crystal Fluid and Glassy−Nematic Oligofluorenes on Coumarin-Containing Polymer Films

Abstract

The orientations of both a nematic liquid crystal fluid and a series of monodisperse glassy−nematic oligofluorenes were investigated on photoalignment films comprising a polymethacrylate backbone with 7-benzoyloxycoumarin pendants. Both classes of liquid crystalline material were found to undergo a transition from a parallel to a perpendicular orientation with reference to the polarization axis of UV-irradiation at a sufficiently high extent of dimerization. The UV−vis and FTIR spectroscopic analyses revealed photostability with irradiation up to a fluence of 10 J/cm2, thereby excluding photodegradation as the basis for the observed crossover. A kinetic model was used to interpret the crossover behavior for irradiation at 25 °C, leading to the conclusion that liquid crystal molecules interact more favorably with coumarin monomers than with dimers. Through thermal annealing above Tg followed by cooling to room temperature, a glassy−nematic pentafluorene film was prepared on a photoalignment film, exhibiting an orientational order parameter comparable to that on a rubbed polyimide film. At an increasing oligofluorene chain length, however, a decreasing orientational order parameter emerged, presumably because of the increased annealing temperature that causes an orientational relaxation on the part of the dimerized coumarin moieties.